1. Field of the Invention
The present invention generally relates to an infrared detector having photocurrent responses from near infrared to very long wavelength infrared at normal and oblique incidences. Specifically, the invention is a quantum-well infrared photodetector composed of group III-V nitrides.
2. Description of the Related Art
Infrared detectors are found in a wide variety of imaging applications, including night vision goggles, surveillance satellites, and seekers. Technical advances continue to increase the resolution and range of infrared systems and to lower their operational costs.
Second-generation infrared systems include two-dimensional focal plane arrays functioning as either photon or heat detectors. Photon detector systems include mercury cadmium telluride (MCT) elements in a pixelized arrangement so as to detect long-wavelength infrared (LWIR, 8–12 μm), and MCT and indium antimonide (InSb) elements in a pixelized arrangement so as to detect medium-wavelength infrared (MWIR 3–5 μm). Heat or thermal detectors include microbolometers and pyroelectric sensors. Such devices do not require cooling and therefore are lighter and less expensive than photon detectors. However, microbolometers and pyroelectric sensors are resolution and range limited.
Third-generation infrared systems are separable into three distinct design approaches, namely, large format two-dimensional focal plane arrays, multi-spectral detection and correlation at two or more wavelengths, and longer wavelength sensing.
Exemplary third generation detectors include multi-spectral MCT, antimonide-based devices, and quantum-well infrared photodetectors (QWIPs). MCT technology has been demonstrated in focal plane arrays having as many as four million pixels sensing infrared wavelengths up to 17 μm. However, large format focal plane arrays suffer uniformity and operability problems in the range of LWIR. Theoretically, antimonide-based devices are wavelength tunable and capable of quantum efficiencies exceeding 80%. However, material and surface problems limit detector performance in practical applications. QWIPs facilitate large format focal plane arrays and multiple spectral detection in the range of MWIR and LWIR. However, arsenide-based devices, such as those described and claimed by Gunapala et al. in U.S. Pat. Nos. 6,734,452 B2 and 6,211,529 B1, have a low quantum efficiency (10–20%), require cooling, and fail to detect infrared around 37 μm.
Both second and third generation devices are unable to absorb normal incident light. Gratings and beveled edges are employed to correct this deficiency. However, both approaches increase complexity and cost and degrade performance by increasing crosstalk.
While nitride-based compositions have been applied to multiple quantum wells in light emitting diodes and lasers, application to far infrared detectors is not found in the related arts. Furthermore, quantum well structures within diodes and lasers are simply too conductive for far infrared detection. For example, a quantum well within a typical diode or laser has an electron concentration exceeding 1018 cm−3, thereby reflecting infrared light at wavelengths above 33 μm. Since wavelengths as high as 100 μm are needed for some far infrared detector applications, it is desired to have a free electron concentration less than 1018 cm−3.
What is currently required is an infrared detector with improved sensitivity and capable of operating at higher temperatures so as to extend the operational range of imaging systems.